Bioinspired energy conversion systems for hydrogen production and storage

Recent developments in photocatalytic hydrogen production by using artificial photosynthesis systems is described, together with those in hydrogen storage through the fixation of CO₂ with H₂. Hydrogen can be stored in the form of formic acid, which can be converted back to H ₂ in the presence of an appropriate catalyst. Electron donor-acceptor dyads are utilized as efficient photocatalysts to reduce methyl viologen (MV²⁺) by NADH (β-nicotinamide adenine dinucleotide, reduced form) analogues to produce the methyl violgen radical cation that acts as an electron mediator for the production of hydrogen. Porphyrin-monolayer-protected gold clusters that enhance the light harvesting efficiency can also be used for the photocatalytic reduction of methyl viologen by NADH analogues. The use of a simple electron donor-acceptor dyad, the 9-mesityl-10-methylacridinium ion (Acr⁺-Mes), enables the construction of a highly efficient photocatalytic hydrogen-evolution system without an electron mediator such as MV²⁺, with poly(N-vinyl-2-pyrrolidone)-protected platinum nanoclusters (Pt-PVP) and NADH as a hydrogen-evolution catalyst and an electron donor, respectively. Hydrogen thus produced can be stored in the form of formic acid (liquid) by fixation of CO₂ with H₂ in water by using ruthenium aqua complexes [Ru^II(η⁶-C ₆Me₆)(L)(OH₂)]²⁺ [L = 2,2′-bipyridine (bpy), 4,4′-dimethoxy-2,2′-bipyridine (4,4′-OMe-bpy)] and iridium aqua complexes [Ir^IIICp*(L) (OH₂)]²⁺ (Cp* = η⁵-C ₅Me₅, L = bpy, 4,4′-OMe-bpy) as catalysts at pH 3.0. Catalytic systems for the decomposition of HCOOH to H₂ are also described. The combination of photocatalytic hydrogen generation with the catalytic fixation of CO₂ with H₂ and the decomposition of HCOOH back to H₂ provides an excellent system for cutting CO ₂ emission.